The objective of this research program is to study the lifetime risk of radiation exposure in infancy and adulthood. Two populations with known x-ray exposures have been followed. One population consisting of almost 3,000 persons treated in infancy with known x-ray doses to the chest (for alleged thymic enlargement) has been followed for 20 years, and the second, 600 women given x-ray treatments to be breast (for acute post-partum mastitis), for ten years. As controls, we use the 5,000 untreated siblings of the thymus-treated subjects and the general population in upstate New York and 550 women with acute post- partum mastitis not treated with x-rays (in New York City) as well as upstate New York women. To date, we have found an increased incidence of cancer in both irradiated series. The persons with chest irradiation have a great excess of malignant and benign thyroid neoplasms as well as a smaller excess of lymphoma. Thyroid cancer shows a distinct age and dose response. The women with breast irradiation have more than doubled the number of cases of breast cancer than the untreated controls. The persons whose thymus glands were exposed in infancy also seem to have a significantly increased incidence of rare illnesses with abnormal immunological features (three times as frequent as that noted in the sibblings). There is experimental evidence in animals indicating that these rare illnesses could be a late consequence of the thymic irradiation. Periodic mail surveys will continue hopefully throughout the lifetime of the irradiated population.